1. Field of the Invention
The present invention relates to an organic electroluminescence display device and an organic electroluminescence display device manufacturing method.
2. Description of the Related Art
Image display devices (organic EL displays) using organic electroluminescence elements (OLED: Organic Light-Emitting Diodes) are well-known as image display devices using current-driven light-emitting elements. Due to such advantages as excellent viewing angle characteristics and low power consumption, such organic EL displays have gained much attention as candidates for next-generation flat panel displays (FPDs).
In organic EL display devices, organic EL elements included in pixels are normally arranged in a matrix. In an organic EL display referred to as a passive-matrix organic EL display, an organic EL element is provided at each crosspoint between row electrodes (scanning lines) and column electrodes (data lines), and such organic EL elements are driven by applying a voltage equivalent to a data signal, between a selected row electrode and the column electrodes.
On the other hand, in an organic EL display device referred to as an active-matrix organic EL display, a thin film transistor (TFT) is provided in each crosspoint between scanning lines and data lines, the gate of a drive transistor is connected to the TFT, the TFT is turned ON through a selected scanning line so as to input a data signal from a data line to the drive transistor, and an organic EL element is driven by such drive transistor.
Unlike in the passive-matrix organic EL display where, only during the period in which each of the row electrodes (scanning lines) is selected, does the organic EL element connected to the selected row electrode emit light, in the active-matrix organic EL display, it is possible to cause the organic EL element to emit light until a subsequent scan (selection), and thus a reduction in display luminance is not incurred even when the number of scanning lines increases. Therefore, since driving with low voltage is possible, reduction of power consumption becomes possible. However, in the active-matrix organic EL display, due to variation in the characteristics of drive transistors and organic EL elements arising in the manufacturing process, the luminance of the organic EL elements are different among the respective pixels even when the same data signal is supplied, and thus there are instances where luminance unevenness, such as a band or unevenness, occurs. Specifically, an error occurs between the voltage-luminance characteristics of each pixel and representative voltage-luminance characteristics that are common among pixels. Here, in the voltage-luminance characteristics of each pixel, the variation with the high gradation region of the representative voltage-luminance characteristics is mainly due to variation in drive transistor mobility, and the variation with the low gradation region of the representative voltage-luminance characteristics is mainly due to variation in drive transistor threshold voltage (Vth).
In response, there is proposed a correction method of correcting the luminance unevenness occurring in an organic EL display in which, by correcting an image signal (data signal), the luminance of the organic EL elements corresponding to the image signal supplied to the respective pixels can be corrected to a predetermined standard luminance (for example, Patent Reference 1: Unexamined Japanese Patent Application Publication No. 2005-284172.
In the correction method of Patent Reference 1, by measuring the luminance distribution or current distribution of at least three gradation levels in each pixel of an organic EL display, it is possible to obtain the gain and offset which are correction parameters for correcting the luminance of the organic EL element corresponding to the image signal supplied to the respective pixels to a predetermined standard luminance, that is, for correcting the drive transistor variation.
Meanwhile, a method of configuring a Vth compensation circuit in a pixel of an organic EL display, and suppressing variation in the characteristics of drive transistors has been proposed (for example, Patent Reference 2: WO/2008/152817).
The correction method of Patent Reference 2 evens-out the display, in other words, suppresses luminance unevenness in an organic EL display by suppressing Vth variation in the TFT characteristics of drive transistors in the initial characteristics and the Vth variation following deterioration over time.
However, the conventional correction methods have the problems described below.
For example, as a correction method of Patent Reference 1, there is a method of obtaining gain and offset, which are correction parameters, using the least-square technique. In this method which uses the least-square technique, multi-gradation level luminance measurement is performed for each pixel, and the gain and offset are obtained using a predetermined calculation method, based on the luminance difference between the luminance of each pixel obtained in each measurement and the representative voltage-luminance characteristics. This method shall be described.
FIG. 1 is a diagram showing a configuration of a conventional luminance measurement system at the time of luminance measurement. FIG. 2 is a flowchart for describing the conventional correction method. FIG. 3 is a graph showing an example of a case where correction parameters are obtained using the least-square technique.
The conventional luminance measurement is performed on each pixel included in a display panel 806 of a conventional organic EL display device 800, for a number of gradation levels that is at least 3 gradation levels and preferably 5 gradation levels or more, using a measuring device 910. Here, the conventional organic EL display device 800 includes the display panel 806 and a control unit 801. Furthermore, the luminance measurement system includes the organic EL display device 800, the measuring device 910, and a correction parameter determining device 900.
The measuring device can measure luminance that is emitted by the pixels included in the display panel 806. The correction parameter determining device 900 is a device which determines the correction parameters, that is, the gain and the offset for correcting the luminance of the pixels included in the display panel 806 to the standard luminance, based on the luminance of each pixel measured by the measuring device 910.
In this luminance measurement system, luminance measurement is performed on each pixel for a number of gradation levels that is 5 gradation levels or more, as shown in FIG. 2. Specifically, a pixel included in the display panel 806 is caused to display (emit light) at a certain gradation level (N gradation level) (S801), and the luminance of the pixel is measured using the measuring device 910 (S802). Subsequently, the data indicating the luminance measurement value for the N gradation level is, for example, stored in a memory or the like of a personal computer (PC) connected to the measuring device 910 or the correction parameter determining device 900 (S803), then S801 to 5803 are repeated up to the number of gradation levels for measurement (S804, S805). When the number of times the luminance measurement is performed reaches the number of gradation levels for measurement (Y in S804), a correction process is performed by the correction parameter determining device 900 (S806). Here, the correction parameter determining device 900 measures, for a certain pixel for example, luminance L1 to L6 at the six points (N=6 is assumed) of voltages V1 to V6 using the least-square technique, and obtains V×1 to V×6 as the correction parameters, as shown in FIG. 3. Subsequently, the correction parameter determining device 900 writes the correction parameters (gain and offset) for each pixel into a memory included in the control unit 801.
However, as described above, in the correction method of Patent Reference 1 which uses the least-square method for example, by nature it is necessary to perform the luminance measurement on each pixel for a number of gradation levels that is at least 3 gradation levels and preferably 5 gradation levels or more, and thus there is the problem of requiring time from the performance of the luminance measurement for each pixel up to the obtainment of the correction parameters. In particular, a very long time is required for the luminance measurement in the low gradation-side.
Furthermore, in an organic EL display, there is a tendency for the occurrence of streaky luminance unevenness in the low gradation regions. The human eye recognizes luminance differences more easily in the low gradation-side than in the high gradation-side. As such, it is preferable that correction precision be higher for the low gradation-side than the high gradation-side. However, normally, the luminance difference between the representative voltage-luminance characteristics and the voltage-luminance characteristics of each pixel increases as one goes further into the high gradation-side, and since the least-square method simultaneously obtains the gain and offset by calculation so that the luminance error in the high gradation-side is minimized, there is the problem that, although the correction error in the high gradation-side can be minimized, the correction error in the low gradation-side becomes big compared to that in the high gradation-side.
On the other hand, in the correction method shown in Patent Reference 2, it is possible to cope with the variation in TFT threshold voltage Vth of the drive transistors by configuring a Vth compensation circuit, but it is not possible to sufficiently correct the variation in TFT mobility. As such, there is the problem that, in the displaying in an organic EL display, luminance unevenness corresponding to the mobility variation remains.